Mechanisms for limiting access to and operability of an automobile came into general use shortly after the general acceptance of the automobile. For example, tumbler locks operable by keys have long been used on doors, glove compartments, trunks, and ignitions. The desirability of eliminating the need for keys also has long been recognized. The Gilmore, U.S. Pat. No. 1,251,365; Chrisman et al, U.S. Pat. No. 1,298,177; Carlson, U.S. Pat. No. 1,587,757; Gibbs, U.S. Pat. No. 2,819,770; and Raju, U.S. Pat. No. 2,964,733, disclose previous proposals for keyless locking systems. None of these proposals is really practical for use in an automobile.
Later attempts to solve this problem used electrical combination locks having push buttons. See, for example, the U.S. Pat. No. 3,353,383 to Fish and to Gaumer, U.S. Pat. No. 3,544,804. Push buttons mounted externally on an automobile are large, expensive, sensitive to environmental factors, and subject to wear. Other recent patents on electrical combination locks include the U.S. Pat. No. 3,024,452; to Leonard, Hevenor, U.S. Pat. No. 3,192,448; and Hinrichs, U.S. Pat. No. 3,691,396, all of which fail to solve the limitations of hardwired logic, mechanical elements, and mechanical switches.
More recent U.S. patents, which have considerably advanced the art, are the Haygood et al, U.S. Pat. No. 4,205,325, and to Ligman et al, U.S. Pat. No. 4,206,491. The Haygood patent discloses a permanent preprogrammed code storage memory and a user programmable code storage memory wherein either code may be used to gain entry to the vehicle and to enable other functions, but the Haygood et al apparatus requires a pushbutton system which necessitates custom bodywork for installation. The Ligman et al patent discloses the use of piezoelectric switches applicable to vehicle locking systems but does not suggest the provision of security alarms. Piezoelectric switches are preferable to mechanical switches in that they are not subject to mechanical wear, are vandal resistant, and can be completely sealed from the environment. On the other hand piezoelectric switches are high electrical impedance devices and thus subject to the problems of electrical interference by extraneous noise signals. Conventional piezoelectric switching techniques require the use of shielded cables between the piezoelectric switches and electronic signal processing units or special signal conversion units located adjacent the piezoelectric switches.
It is not new, of course, to equip a vehicle with an alarm system. The prior art contains numerous examples of alarms which may be activated in the event a vehicle is entered or attempted to be started by an unauthorized person. Those systems, however, are believed to be limited to alarms and associated equipment which, although responsive to unauthorized entry and starting attempts, are really independent units which must be preset by an operator, independently of the vehicle's locking mechanism, so as to be conditioned for operation upon the occurrence of a predetermined event, such as tampering with the vehicle's ignition by an unauthorized person.